1. Field of the Invention
This invention relates to a color image reading method and apparatus in which the image of an object is read as image information having color information by image pickup means comprising a CCD (charge coupled device).
2. Description of the Prior Art
In recent years, various types of image transmitting or processing apparatus have been popularized along with the development of the electronic technique. In such type of apparatus, a portion for converting the image of an object into electrical signals, i.e., an image reading apparatus, is indispensable. In such an image reading apparatus, as typically seen in many types of apparatus including facsimile, use is widely made of a method in which a so-called line sensor comprising a plurality of picture elements arranged in the form of a row is used and this line sensor is moved in a direction substantially orthogonal to the direction of the row relative to the image of an object formed by an imaging optical system (minor scanning) while the line sensor is major-scanned, whereby the image is read.
Now, for the image reading apparatus, it is desired that the reading time be as short as possible and the energy required for illuminating the object be small. For this purpose, the imaging optical system used must be as bright as possible. Also, as viewed from the viewpoint of demand, the color reading function will be required of image sensors in the future.
As a color image reading apparatus, there is known one in which a plurality of rows of line sensors corresponding to lights of different wavelength ranges are provided on the imaging plane of an object, as shown, for example, in Japanese Laid-open patent application No. 102614/1977.
However, the color image reading apparatus according to the prior art has suffered from a problem that in the imaging optical system as described above, there is a tendency that the imaging position is often varied by the wavelength of light.
FIG. 1 of the accompanying drawings is a schematic view for illustrating the conventional reading method in a case where an array of light-converging fiber (tradename: SELFOC; produced and sold by Nippon Sheet Glass Co., Ltd.) is used in the imaging optical system. The light-converging fiber array 101 causes the image of an original 102 to be formed at a position 103. The then distance between the original 102 and the image 103 thereof is L. It is also to be understood that the distance between one end of the light-converging fiber array 101 and the original 102 is equal to the distance between the other end of the light-converging fiber array 101 and the image 103.
The result of MTF (modulation transfer function) relative to the spatial frequency of the original examined at the position of the image 103 in FIG. 1 is shown in FIGS. 2 and 3 of the accompanying drawings.
In FIG. 2, reference numeral 201 shows the result measured by red light having a central wavelength .lambda.=0.620 .mu.m and with L=20 mm, reference numeral 202 shows the result measured by green light having a central wavelength .lambda.=0.535 .mu.m and with L=18 mm, and reference numeral 203 shows the result measured by blue light having a central wavelength .lambda.=0.445 .mu.m and with L=16 mm, and in each case, MTF has been good.
In FIG. 3, L=18 mm for all of the lights of various wavelength ranges in FIG. 2. In FIG. 3, MTF 302 for green light is good, while MTF 301 for red light and MTF 303 for blue light are extremely reduced. This is because L=18 mm is not appropriate for red light and blue light.
Accordingly, if an image sensor is provided at the imaging position for light of a particular wavelength (e.g. green light), the resolution is remarkably reduced for lights of other wavelengths (e.g. red light and blue light).
To prevent this, an achromatic lens may be employed in the imaging optical system, but such a lens is expensive. The light-converging fiber array as previously described cannot be used. An alternative method using a discrete imaging optical system for light of each wavelength range is liable to lead to a complicated and bulky construction. These problems have formed great impediments in realizing a compact and inexpensive color image reading apparatus of high resolution.